In their maximal generality, simplification operations in HOL Light (as invoked
by SIMP_TAC) are controlled by a `simpset', which may contain conditional and
unconditional rewrite rules, conversions and provers for conditions, as well as
a determination of how to use the prover on the conditions and how to process
theorems into rewrites. A call ss_of_congs thl ss adds thl as new
congruence rules to the simpset ss to yield a new simpset. For an
illustration of how congruence rules can be used, see extend_basic_congs.